In recent years, researches have been in progress for polymer electrolyte fuel cells employing a proton-conductive polymer membrane as an electrolyte. Such polymer electrolyte fuel cells have characteristics such that they are operable at a low temperature, provide a high output density and can be small-sized, and thus, they are expected to be prospective in their use as e.g. power sources for vehicles.
As an electrolyte membrane for polymer electrolyte fuel cells, a proton conductive ion exchange membrane having a thickness of from 20 to 200 μm is commonly used. Particularly, a cation exchange membrane made of a perfluorocarbon polymer having sulfonic groups (hereinafter referred to as a sulfonic perfluorocarbon polymer) is excellent in basic properties and thus is widely studied.
As a method for reducing the electric resistance of such a cation exchange membrane, the sulfonic group concentration may be increased, or the membrane thickness may be reduced. However, if the sulfonic group concentration is substantially increased, the mechanical strength of the membrane tends to decrease, or the membrane tends to be susceptible to creeping during the operation of the fuel cell for a long period of time, thus leading to a problem such as a decrease in the durability of the fuel cell. On the other hand, if the membrane thickness is reduced, there will be a problem such that the mechanical strength of the membrane tends to be low, and in a case where a membrane-electrode assembly is to be prepared by bonding the membrane to a gas diffusion electrode, such processing tends to be difficult or the handling efficiency tends to be poor.
Further, upon absorption of water, the electrolyte membrane tends to have the size increased in the longitudinal direction of the membrane, whereby various problems are likely to be brought about. For example, when a membrane-electrode assembly is assembled in a fuel cell, which is then operated, the membrane will be swelled by water formed by the reaction or by steam, etc. supplied together with the fuel gas, whereby the size of the membrane will be increased. As the membrane and the electrode are bonded, the electrode usually follows the dimensional change of the membrane. The membrane-electrode assembly is usually restricted by a separator or the like, having grooves formed as flow channels for a gas, whereby the increase in the dimension of the membrane creates “wrinkles”. And, such wrinkles will fill in the grooves of the separator to hinder the gas flow.
As a method to solve such a problem, a method has been proposed wherein a porous media made of polytetrafluoroethylene (hereinafter referred to as PTFE) is impregnated with a sulfonic perfluorocarbon polymer (Patent Document 1). However, the porous media of PTFE is relatively soft due to the material, whereby the reinforcing effect is inadequate, and the above problem has not yet been solved. Further, a method of filling an ion exchange resin to a porous media made of a polyolefin, has also been proposed (Patent Document 2), but the chemical durability was inadequate, and there was a problem in the stability for a long period of time.
Further, as another reinforcing method, a method of employing fluororesin fiber has been proposed. There is a method for producing a cation exchange membrane reinforced with a reinforcing material of a fibril-form fluorocarbon polymer (Patent Document 3) or a method for preparing a polymer membrane reinforced by short fibers of a fluororesin (Patent Document 4). In these final products, the reinforcing materials themselves were not particularly positively entangled or bonded, whereby the reinforcing efficiency was poor, and it was necessary to incorporate a relatively large amount of such reinforcing materials. In such a case, processing to thin film tends to be difficult, and an increase in the membrane resistance was likely to be brought about.
Further, Patent Document 5 proposes an electrolyte membrane for polymer electrolyte fuel cells, reinforced by a fluorinated fiber sheet wherein fluorofibers being non-continuous short fibers having a length of at most 15 mm are bonded to one another by a binder such as viscose, carboxymethylcellulose or polyvinyl alcohol. Such a binder is an impurity for the electrolyte membrane for fuel cells, and the remaining binder substantially impairs the durability of the fuel cells. Further, in the proposal, fibers having a relatively large fiber diameter i.e. a fiber diameter of 15 μm, are employed, and in order to realize sufficient bonding of the fibers to one another, the reinforcing media is required to have a thickness of few times the fiber diameter, whereby it is considered that an increase in the membrane resistance could easily be brought about. Further, in the method of processing the non-continuous short fibers by a method such as a paper making method, there will be a problem in making the membrane to be thin, such that with extremely thin fibers, handling is practically difficult.    Patent Document 1: JP-B-5-75835 (claims)    Patent Document 2: JP-B-7-68377 (claims)    Patent Document 3: JP-A-6-231779 (claims)    Patent Document 4: WO04/011535 (claims)    Patent Document 5: JP-A-2003-297394 (claims, paragraphs 0012 and 0026)